This invention relates to a steering control system for crawler tractor type vehicles, and more particularly to hydraulic controls of a steering control system for endless track type tractors or crawler tractor type vehicles having dual power trains.
In the vehicles having dual power trains, the power developed by the engine is transmitted through a power distributor unit to respective torque converters provided in the left and right power transmission trains, and then through a universal joint to each of the input shafts of the left and right transmission gear units. The power thus transmitted will actuate a transmission control valve in response to the fluctuations of the load, and the power selected by the clutch located in the transmission gear unit is decelerated by a transfer device, and then transmitted to a bevel pinion. The power which is, in turn, transmitted to the rearward of the vehicle is converted from the longitudinal direction to the direction at right angles thereto by means of a bevel gear associated with the bevel pinion and mounted on the transverse shaft. The power transmitted to the transverse shaft is transmitted to the sprockets while it is being decelerated by pinions of the final drive. In the intermediate portion of the transverse shaft, there is mounted a center clutch unit which divides the shaft into two parts, namely, left and right parts. By engaging or disengaging a center clutch in the center clutch unit, the vehicle can be driven straight or be turned. Further, because the left and right transverse shafts can be rotated by coupling the center clutch, it becomes possible to prevent the vehicle from its zigzag movement due to unbalance in the power transmitted to the left and right power trains, and also it becomes possible to prevent the vehicle from running obliquely due to a difference in the running resistances to which the left and right power transmission trains are subjected during operations such as side cutting in which uneven loads are applied to them.
In the prior art steering control system for vehicles having left and right power transmission trains disclosed, for example, in the specification of the U.S. Pat. No. 3,785,450, three power units, namely, the transmission gear unit, steering brake units and a center clutch unit have been independently controlled by means of the left and right steering levers and the brake pedals, respectively. Therefore, a great many directly connected linkages and interlocking linkages are required to be installed, and further in order to perform a predetermined operational sequence between the power units, it is required to adjust many parts to be assembled such as cam mechanisms, elongated holes and loose springs, etc. For this reason, a great deal of adjustments jobs are required not only when the power units are assembled at factories, but also when they are worn out due to their use for an extended period of time and replaced with new ones at work sites. For examples, upon adjustments of the connection of linkages associated with the center clutch and the transmission gear units and adapted to be actuated by means of the steering lever, if the linkages are connected so as to reverse their operational sequence, then the load applied on the center clutch will increase to cause it to slide which results in early wear-down of it. Further, upon adjustments of the connection of linkages associated with the transmission gear units and the steering brake units and adapted to be operated by the steering lever, if the linkages are connected so as to reverse their operational sequence, then the load will be concentrated on the brake thus causing an early wear-down thereof. Still further, upon adjustments of the connection of linkages associated with the steering brake units and the center clutch unit, if the aforementioned poor adjustments are made, then it becomes impossible to effect the turning operation of the vehicle. Still further, regarding brake linkages actuated by the brake pedals, if the synchronized operations of the left and right linkages are not made positively, then the load is concentrated on only the brake on one side when the brake pedal is depressed, while the other brake will be put under half brake condition thus causing insufficient braking force and early wear-down of the brake on one side.